When you play sports outside, you may accidently fall and scratch your knee or elbow, shedding a few drops of blood. Usually this is not a reason to be concerned. You wash your cut with some water, apply an anesthetic spray or ointment and stick on a bandage. The reason most of us don’t need to worry about bleeding to death after sustaining minor injuries is our body’s ability to initiate a defense response called hemostasis. Hemostasis means our blood clots and forms a scab to begin the healing process.

The process of hemostasis involves three steps:

• Vascular spasm is the constriction of the injured blood vessel.
• Aggregation is when the blood cell fragments, called platelets, form a plug that fills the gap in a broken vessel.
• Blood coagulation is what we most often call clotting, a multistep process that involves many chemical reactions.

Clotting factors—proteins in the blood that help control bleeding—act as enzymes, each one converting the next factor from an inactive to an active form. We can compare the clotting cascade to a falling row of dominoes. The cascade will continue until sticky substances called fibrin polymers form to trap the blood cells and platelets. The formation of fibrin is what effectively stops the bleeding.  

Lab tests that measure bleeding time can determine whether the blood coagulation process occurs properly. For example, in a healthy person, the bleeding after a fingerpick test should stop within two to three minutes.

When dealing with processes as complex as the coagulation pathway, any abnormal clotting factor can shut the entire cascade down. This occurs when a person has hemophilia, a group of hereditary diseases in which one of clotting factors is lacking. The most common type of hemophilia, called hemophilia A or factor VIII deficiency, is caused by the absence of clotting factor VIII and is responsible for 83% of hemophilia cases.

If you learned genetics in school, you may already know about sex-linked diseases. Hemophilia is one of them, as it affects predominately males, who have only one copy of the X chromosome. Females have two X chromosomes, and even if one of them has a mutated gene that causes a missing clotting factor, half of the normal level of clotting factor is usually enough to keep the clotting process normal and to prevent symptoms of hemophilia. As a result, females with one mutated gene copy are called carriers—they do not develop hemophilia themselves but can pass it to their sons. However, female carriers of hemophilia can also have bleeding problems, sometimes showing up only when they begin their menstrual cycles or go through childbirth. These women are often described as having mild hemophilia. The most well-known case of a carrier for hemophilia is Queen Victoria, who passed the gene through her many children to royalty all over Europe.

Before techniques were developed to replace missing clotting factors in people with hemophilia more than half of the people with this condition died in early childhood. Any type of physical activity caused muscle and joint bruising and debilitating pain. Serious injuries, such as head bleeds, could be fatal. However, advances in medical care and treatment now allow people with hemophilia to live a normal, full life.